550 Access denied - Invalid HELO name (See RFC5321 4.1.1.1)

I'm not a regular forum user but this time I had no other option. I'm facing this problem since I migrate from shared hosting to a VPS with WHM / Cpanel and sometimes, just in SOME HOSTS, when i send an email, i get this error from from my own VPS, by "Mail delivery System"

I've been fighting against this and i already turn off "Require RFC-compliant HELO" and even "Require HELO before MAIL" but nothing seems to work. I disabled DKIM, SPF, enabled again, and nothing works. How can I solve this? My clients are angry (I understand that) and there's nothing more I can do by now.

The problem is not only on Outlook (yes, i tried "my server requires authentication"). Even when I try to send a email to those specific webmails by Cpanel software I receive that error.